Scientific American Supplement, No. 492, June 6, 1885 online

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f/8. For example, a Ross half plate rapid symmetrical has a focal length
of 7ВЅ in.; for convenience reduce this to sixteenths=120. A diaphragm
measuring seven sixteenths will give the fraction f/17. Now let us see if
any of these stops correspond with Mr. Burton's. The first two in his
table will only be found in portrait lenses, but we shall probably find
one to correspond with the third, if we are using a doublet lens; with a
single lens we won't find any so large. Having picked out those that
correspond, and filled in the exposure for them, we have now to deal with
the odd sizes. Here is one, f/27, which is just half way between No. 16
and No. 32, but a moment's thought will show that as the exposure
increases as the square of the diameter, it won't do to take the exposure
half way between the two.

We have another factor to consider now: that is, the rapidity of the
plate. If you use plates by a maker who has a name to sustain, you may be
pretty confident that they are of fairly uniform rapidity, so after you
have got into the way of working any particular brand, the best thing you
can do is to stick to it. The exposures in our table are for plates of
medium rapidity in good spring light. In my own experience I find that
they just suit "thirty times" plates, or fifteen on the sensitometer; but
then I like a full exposure with slow development, and I know that others
find these exposures just right for "twenty times" plates developed in
the usual way. The most rapid plates in the market will not be overdone
with half the given exposures. It must always be borne in mind that an
error of a fraction of a second in either direction may be corrected in
development, and it is impossible to make a very serious error if you
refer to the table.

We come now to the light. If you depend on the eye entirely in judging
the quality of the light, it will sometimes play you tricks. The rays
which are most active on the plates are those which have the least effect
on the eye. We can, however, by chemical means arrive at an exact
estimate of the active power, and for this purpose an actinometer is
used. This is simply an arrangement whereby a piece of sensitized paper
is exposed and allowed to darken to a standard tint, and by the time it
takes to reach that tint the value of the light is judged. Capt. Abney
has, however, pointed out that ordinary sensitized paper is not suitable
for bromide plates, since there are conditions of light in which the
plates will be fairly rapid while the paper will be very slow. He gives a
formula for a bromide paper, which is treated with tannin in order to
absorb the bromine set free during exposure, otherwise the darkening
would be very slight. I used this paper for a while, but found it rather
slow. The tannin also turned brown on keeping for a week or so. I then
made some more, substituting for tannin potassium _nitrite_ (not
nitrate), which is colorless. This was an improvement, but still it was
just slow enough.

However, noticing in Capt. Abney's article the statement that the bromide
of silver should be as nearly as possible in the same state in the paper
as in the plate, I thought "Why not Morgan's paper?" This, of course, is
just bromide emulsion on paper, and if, as I suspect from its color, it
contains a trace of iodide, why, so do most commercial plates. A sheet of
this paper cut into strips, soaked for ten minutes in a fifteen-grain
solution of potassium nitrite, and dried, gives a sensitive paper which
darkens with great rapidity to a good deep tint, and keeps indefinitely.
Here is some prepared last summer, which is still quite good. To use this
paper make a little box so that a little roll of it can be stored in one
end, and drawn forward as required beneath a piece of glass.

Bearing in mind that your table of exposures is calculated for the best
spring light, go to the country some bright day next month with
note-book, actinometer, and the necessary appliances for exposing a few
plates. Select, say, an open landscape, and use your smallest stop. When
all ready to expose, get out your actinometer and expose it to the
reflected light of the sky for ten seconds (if the sun is shining, turn
your back to it, and keep the actinometer in your own shadow); then put
it in your pocket, expose a plate according to your table, and in case
the light or plate should not be just in accordance with the conditions
under which the table was prepared, expose other two plates, one a little
less and one a little more than that first exposed. Then note down
everything you have done - kind of view, stop, speed of plate, exposure of
each plate, and length of exposure of actinometer.

When you get home, the first thing to do is to get hold of a paint box
and paint the underside of the glass of your actinometer to match the
darkened paper. Do this by gas light. Then scrape away a little of the
paint, so as to let a strip of the paper be seen below it. After this
develop your three plates with a developer of normal strength, and see
which is best. If you have chosen a really bright spring day, and are
using plates of medium rapidity, you will most likely find that exposed
according to the table just about right.

Now let us see how we can use these aids in our field work. We have
ascertained the correct exposure with a given stop on one class of view,
with light of a given quality, but now suppose all these conditions
altered. Let the view have heavy foliage coming close up to the camera,
the stop be a size larger than that used in our first experiment, and the
day rather dull. The table tells us what the exposure would be with this
stop on this view, on a bright day; and if the actinometer take twenty
seconds to reach the painted tint, then we must double the exposure given
in the table.

You may sometimes find that the actinometer indicates a very different
exposure from what the eye would lead you to expect. For instance, one
day last September I went to Bothwell Castle, to get a picture I knew of
in the grounds. It was one of those strange yellow days we had then, and
the sun, though shining with all his might, was apparently shining
through orange glass. The actinometer indicated an exposure of thirty
seconds where in good light one would be right. I was rather incredulous.
Thirty seconds in broad sunshine! However, I gave this exposure, but for
my own satisfaction I gave another plate fifteen seconds only.

On developing, the latter was hopelessly underexposed while that having
thirty seconds gave a negative which furnished one of my exhibition

I have shown you how to reduce the quality of the light to a certainty,
also how to reduce to rule the exposure with different lenses and stops
on certain classes of subjects, and it remains with you only to guess
correctly to what class the view you wish to take belongs; I can assure
you from my own experience that there is enough uncertainty about that
point to prevent good negatives ever becoming monotonous.

The only aid I can suggest in this case is the continual use of a
note-book. Note every plate you expose, and when you have a failure be
careful to record the fact, and you will gradually find these accumulated
notes becoming a great help in cases of doubt. One hint I can give to
beginners is that a great number of the pictures to be met with in this
part of the country are intermediate between "Open Landscape" and
"Landscape with heavy foliage in foreground;" and it is scarcely needful
to say that if you are in doubt, let the exposure be rather too much than
too little; you _may_ make a negative of an overexposed plate, but never
of an underexposed one.

* * * * *


[Footnote: Read at the stated meeting of the Franklin Institute, March
18, 1885.]


It is well known that the ordinary photographic processes do not
reproduce colors in the true proportion of their brightness. Violet and
blue photograph too light; green, yellow, orange and red, too dark. For a
long time it was believed to be impossible to remedy this defect; and
even when it became known that bromide of silver could be made more
sensitive to yellow and red by staining it with certain dyes, the subject
received very little attention, because it was also known that the
increase of sensitiveness was too slight to be of practical value in
commercial photography.

Dr. H.W. Vogel, who was one of the first, though not the first, to devote
attention to this subject, announced, in 1873, that he had succeeded in
making a yellow object photograph lighter than a blue or violet one, by
using a silver-bromide plate stained with coraline, and exposed through a
yellow glass. The plate showed no increased sensitiveness to red, and the
experiment, although of considerable scientific interest, did not
indicate a practically useful process.

In the spring of 1878 I became interested in this subject, and tried to
discover a method of producing plates which should be sensitive to all
colors, and capable of reproducing them in the true proportion of their
brightness. I commenced by trying nearly all the color sensitizers which
had already been suggested, in order to learn which was the best, and
then, if possible, _why_ it was the best, as a guide to further research.
Chlorophyl was the only thing I tried which was sufficiently sensitive to
red to offer any encouragement in that direction; but the solution which
I obtained was weak and unstable, and far from being a satisfactory color
sensitizer. Hoping to obtain a better solution with which to continue my
experiments, I made extracts from many kinds of leaves, and found that a
solution from blue myrtle leaves looked better and kept better than any
other, and when it was applied to the silver-bromide plates they became
remarkably sensitive, not only to all shades of red, but also to orange,
yellow, and green. By placing in front of the lens a color-screen
consisting of a small glass tank containing a weak solution of bichromate
of potash, to cut off part of the blue and violet light, I obtained, with
these chlorophyl plates, the first photographs in which all colors were
reproduced in the true proportions of their brightness. But my chief
desire at that time was to realize a method of producing from any object
in colors a set of three negatives, in one of which the shadows should
represent the blue of the original, in another the yellow, and in another
the red, in such a manner that transparent pigment prints from these
negatives - blue, yellow, and red - would, when superimposed on a white
surface, represent not only the lights and shadows, but also the colors
of the object. This had already been attempted by others, who failed
because their plates were not sufficiently sensitive to red and yellow.

Having succeeded perfectly in my undertakings, I published my discovery
in 1879,[1] explaining how to prepare and use the chlorophyl plates, in
connection with the yellow screen, for the purpose of securing correct
photographs of colored objects.[2]

[Footnote 1: _Philadelphia Photographer_, December, 1879, p. 365.]

[Footnote 2: I intended this publication to be a very full and explicit
one, and it was sufficiently so to be perfectly understood by most who
saw it; but some may think I did not sufficiently emphasize the
importance of using the particular kind of chlorophyl which I mentioned.
In a brief communication to the editor of the _Photo. News_, in 1883, I
described some experiments with eosine as a color sensitizer, and then
called attention to the superiority of blue-myrtle chlorophyl for this
purpose, stating that I had not been able to secure such results with any
other kind of chlorophyl, and that a fresh solution from fresh leaves
must be used to secure the greatest possible degree of sensitiveness. See
_Photo. News_, Nov. 1883, p. 747.]

So far as I know, nobody tried the process. Nearly five years later Dr.
Vogel announced that, after eleven years of investigation, he had at last
realized a successful process of this character, and that this new
process of his was the "solution of a problem that had long been
encompassed with difficulty." This publication attracted a great deal of
attention, and gave me occasion to again call attention to my process,[3]
and point out that it was not only the first practical solution of this
problem, but the only truly isochromatic process ever discovered. Dr.
Vogel's new process was not only no better in any respect, but the plates
were insensitive to scarlet and ruby-red, and therefore would not
photograph all colors in the true proportion of their brightness.

[Footnote 3: _Photo. News_, London, September 5, 1884, p. 566, and _Year
Book of Photography_ for 1885, p. 111.]

My method consists in treating ordinary collodio-bromide emulsion plates
with blue myrtle chlorophyl solution, exposing them through the yellow
screen, and then developing them in the usual manner. The emulsion which
I have employed is made with an excess of nitrate of silver, which is
afterward neutralized by the addition of chloride of cobalt; it is known
as Newton's emulsion. I now prepare the chlorophyl from fresh blue myrtle
leaves, by cutting them up fine, covering with pure alcohol, and heating
moderately hot; the leaves are left in the solution, and some zinc powder
is added, which helps to keep the chlorophyl from spoiling. I have a
bottle of this solution which was prepared about six months ago, and now
appears to be as good as when first made.[4] A glass plate is flowed with
the emulsion, and as soon as it has set, the chlorophyl solution is
applied for a few seconds, after which the plate is washed in pure water
until smooth, when it is ready for exposure.

[Footnote 4: I originally recommended chlorophyl extracted from dried
leaves, because I had not yet learned how to preserve the solution for
more than a few weeks; and at some seasons it would be difficult, if not
impossible, to obtain fresh leaves. The tea organifier which I
recommended is also a color sensitizer, and when it is used in connection
with the chlorophyl from dried leaves the plates are as sensitive to red
as can be safely prepared and developed in the light of an ordinary
photographic "dark-room." Plates prepared with chlorophyl from fresh
leaves do not require treatment with the tea organifier to secure this
degree of sensitiveness. Recently I have used the tea organifier and some
other sensitizers, in connection with the solution from _fresh_
myrtle-leaves, and in this way have produced plates having such an
exalted color sensitiveness as to be unmanageable in ordinary "dark-room"
light. Possibly, such plates might be prepared and developed in total
darkness, by the aid of suitable mechanical contrivances, but I am not
sure that they would work clear even then, because they appear to be
sensitive to heat as well as to light.]

My color-screen consists of a small plate-glass tank, having a space of
3/16 of of an inch between the glass, filled with a solution of
bichromate of potash about one grain strong. I place the tank in front of
the lens, in contact with the lens-mount. The advantage of this tank and
solution is that it can be more easily obtained than yellow plate glass,
and the color can be adjusted to meet any requirement.

The plates require about three times as much exposure through the yellow
screen as without it, and may be developed with the ordinary alkaline


In order to illustrate the value of this process, I made two photographs
of a highly-colored chromo-lithograph, representing a lady with a bright
scarlet hat and purple feather, a yellow-brown cape and a dark-blue
dress. One, by the ordinary process, represents the blue as lighter than
the yellow-brown, the bright scarlet hat as black, and the purple feather
as nearly white. The other, by the chlorophyl process, reproduces all
colors in nearly the true proportion of their brightness, but with a
slight exaggeration of contrast produced purposely by using a too-strong
color solution in the small tank.

I also made two landscape photographs, one by the ordinary process, and
the other by the chlorophyl process, exposing them simultaneously. In the
ordinary photograph, distant hills are lost through overexposure, yet the
foreground seems underexposed, and yellow straw-stacks and bright autumn
leaves appear black. In the chlorophyl photograph, the distant hills are
not overexposed, nor is the foreground underexposed; the yellow
straw-stacks appear nearly white, and bright autumn leaves contrast
strongly with the dark green about them.

To test the relative color-sensitiveness of plain emulsion plates, plates
stained with eosine, and plates stained with the blue-myrtle chlorophyl,
I exposed one of each kind through the same yellow screen, giving each
five minutes exposure, on the same piece of copy, which was the
chromo-lithograph already described. The plain emulsion plate showed only
the high lights of the picture, after prolonged development. The eosine
plate was underexposed, but brought up everything fairly well except the
scarlet hat, which came up like black. The chlorophyl plate was
overexposed, brought out all colors better than the eosine plate, and
gave full value to the bright scarlet of the hat, the detail in which was
beautifully rendered.

Dr. Vogel advanced the theory that silver-bromide is insensitive to
yellow and red, because it reflects or transmits those colors; and that
it becomes sensitive when stained, because of the optical properties of
the dyes. He afterward admitted that only such dyes as are capable of
entering into chemical combination with the silver-bromide proved capable
of increasing its sensitiveness to color, but he held to the theory that
the optical properties of the compound were the cause of its

I have shown that the color-sensitiveness can be produced by treatment
with an organic compound which has none of the optical properties
characteristic of dyes; and that chlorophyl, which absorbs only red
light, greatly increases the sensitiveness also to yellow and green.
There is, therefore, good reason to doubt if the color-sensitiveness is
ever due to the optical properties of the dye or combination.

Attempts have been made to produce isochromatic gelatine dry plates
which, while many times more sensitive to white light than my chlorophyl
plates, shall also show the same relative color-sensitiveness. Such
plates would be very valuable but for one fact: it would be necessary to
prepare and develop them in almost total darkness. Gelatine bromide dry
plates extremely sensitive to yellow, but _comparatively insensitive to
red_, might be used to advantage in portrait and instantaneous
photography, because they could be safely prepared and developed in red
light; but when truly isochromatic photographs are required, the time of
exposure must be regulated to suit the degree of sensitiveness to red,
which cannot safely be made greater than I have realized with my
chlorophyl process.

* * * * *


The effect of the unequal expansion of paper, when wetted, in causing
distortion of the photographic image impressed upon it, has, in the case
of ordinary photographs upon albumenized paper, been well recognized; but
the extent to which such distortion may exist under different treatment
is worthy of some special consideration, particularly with reference to
the method of printing upon gelatinized paper, which has been thought by
some likely to supersede the method now usually employed with albumenized

When a print upon the ordinary photographic (albumen) paper is wetted,
the fiber expands more in one direction than in the other, so that the
print becomes unequally enlarged, very slightly in one and much more so
in the other way of the paper. When the paper is dried without any strain
being put upon it, the fibers regain very nearly their original
dimensions and position, so that the distortion which has existed in the
wet condition nearly disappears.

If the photograph is cemented, while in the expanded condition, upon a
rigid surface, the distortion then existing is fixed, and rendered
permanent. Such a cementation or method of mounting is that which has
been generally adopted, and the consequence has been that every now and
then complaints have justly been made of the untruthfulness - owing to
this particular distortion - of photographs; productions whose chief merit
has often been asserted to consist in their absolute truthfulness. This
distortion is very manifest when, in a set of portraits, some of the
prints happen to have been made in one direction of the paper, and others
with the long grain the other way. I have known a case where a proof
happened to increase the face in width, and all the other prints
increased it in length. Of course, neither was correct, but the proof had
been accepted and liked, and the remainder of the set had to be reprinted
with the grain of the paper running in the same direction as that in the
first one which had been supplied.

Another evil arising from mounting prints while expanded with moisture
is, that in drying the contraction of the paper pulls round the card into
a curved form and although by rolling this curvature may be temporarily
got rid of, the fiber of the paper is in a strained condition, and the
bent state of the mount is, sooner or later, renewed thereby.

To remedy these evils it has been proposed to mount the print when dry,
by forcible pressure against a slightly damped card, the back of the
print having been previously coated with a cement and dried. This plan
is, to a great extent, successful; but that it does not give absolute
immunity from distortion is, I think, evident from the following
consideration. The prints, after being mounted a few days, will show a
certain tendency to curl inward. This curling, I take it, is a measure of
the strain upon the print, produced by the more complete return to its
original dimensions of the paper photograph. Probably it would be well to
keep the prints a few days after drying, or to subject them to
alternations of damp and dryness, in order to facilitate this complete
return before being placed upon the card. The evil of distortion is,
however, very slight - perhaps imperceptible - compared with that existing
when the prints are mounted wet. I may mention, _en passant_, that I have
found gum much more satisfactory as a mountant than starch paste in what
is known as the "dry mounting" system.

The paper which has recently been introduced for producing prints by
development upon a gelatine surface does not generally, when dried in the
usual way, give so good or so brilliant a surface as that of albumenized
paper; but on the other hand it is very easy with it to obtain what is
called an enamel surface, by simply allowing it to dry in contact with a
prepared surface of glass. This method of finishing has therefore been
much recommended and adopted, but without consideration of the effect of
distortion in connection with it. In an ordinary photograph the print is
mounted damp, but in the case of a print squeegeed on to the glass, the
paper is saturated and thoroughly swollen, and the use of the squeegee
strains it out to its fullest extent. By drying in the position in which
it has been held by contact with the glass, the distortion becomes fixed,
and if the print is mounted while in this state the distortion is made
permanent. How long the strain and distortion remain in an unmounted
print, and whether by time and alternations of moisture and dryness the
strain would be lost, and if so, whether the brilliant enamel surface
would go at the same time, are questions worthy of further investigation
and discussion.

For mounting prints upon developed gelatine paper, it has been
recommended to cement the edges only, so as to leave the greater part of
the print with its enamel surface. This plan is unsatisfactory, for two
reasons, besides the objection on the ground of distortion. There is a
rough-looking margin which spoils the continuity of appearance,
especially (as in the specimens I have seen) where the line of cement is
not kept at an exact width, but encroaches here and there.

Secondly, the print, from not being attached to the mount all over, is
apt, especially when in a large size, to be somewhat wavy and wanting in

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Online LibraryVariousScientific American Supplement, No. 492, June 6, 1885 → online text (page 4 of 10)